Bell's theorem for general N-qubit states

We derive a single general Bell inequality which is a sufficient and necessary condition for the correlation function for N particles to be describable in a local and realistic picture, for the case in which measurements on each particle can be chosen between two arbitrary dichotomic observables. We also derive a necessary and sufficient condition for an arbitrary N-qubit mixed state to violate this inequality. This condition is a generalization and reformulation of the Horodecki family condition for two qubits.     

Thyristor Current Source with an Improved Power Factor

This paper describes the steady-state and dynamic characteristics of a microprocessor-controlled current source. The source is based on an eight-thyristor phase commuted bridge which gives an improved power factor and decreased harmonic distortion. Limiting curves for continuous conduction are presented together with line and thyristor currents in a delta-y input transformer bank. The system's salient characteristics, such as an equidistant firing scheme, a compensator of line voltage variations, and the elimination of a filter in the current feedback loop are described. The transfer function of the current source, with the thyristor switching delays included, is derived and is verified experimentally. It is practically demonstrated that a very fast response is attainable even with a microprocessor-controlled inner current loop.     

Bell's inequalities and quantum communication complexity

We prove that for every Bell's inequality, including those which are not yet known, there always exists a communication complexity problem, for which a protocol assisted by states which violate the inequality is more efficient than any classical protocol. Violation of Bell's inequalities is the necessary and sufficient condition for quantum protocol to beat the classical ones.     

Performance Analysis of Dual-Branch Selection Combining Over Correlated Rician Fading Channels for Desired Signal and Cochannel Interference

System performances of dual selection combining over fading channels are analyzed. Fading between the diversity branches and between interferences is correlated and Rician distributed. Infinite series expressions for the probability density function, and the cumulative distribution function of the output signal-to-interference ratio are derived, which is the main contribution of this paper. Outage probability and the average bit error probability for noncoherent modulation schemes are also presented. Numerical results, presented in this paper, point out the effects of fading severity and correlation on the system performances.     

Quantum phase slips in superconducting wires with weak inhomogeneities

Quantum phase slips are traditionally considered in diffusive superconducting wires which are assumed homogeneous. We present a definite estimate for the amplitude of phase slips that occur at a weak inhomogeneity in the wire where local resistivity is slightly increased. We model such a weak link as a general coherent conductor and show that the amplitude is dominated by the topological part of the action. We argue that such weak links occur naturally in apparently homogeneous wires and adjust the estimate to that case. The fabrication of an artificial weak link would localize phase slips and facilitate a better control of the phase-slip amplitude.     

Necessary and sufficient condition for nonzero quantum discord

Quantum discord characterizes "nonclassicality" of correlations in quantum mechanics. It has been proposed as the key resource present in certain quantum communication tasks and quantum computational models without containing much entanglement. We obtain a necessary and sufficient condition for the existence of nonzero quantum discord for any dimensional bipartite states. This condition is easily experimentally implementable. Based on this, we propose a geometrical way of quantifying quantum discord. For two qubits this results in a closed form of expression for discord. We apply our results to the model of deterministic quantum computation with one qubit, showing that quantum discord is unlikely to be the reason behind its speedup.     

Effect of Microdiversity and Macrodiversity on Average Bit Error Probability in Gamma‐Shadowed Rician Fading Channels

In this letter, we analyze the error performance of a mobile communication system with microdiversity and macrodiversity reception in gamma‐shadowed Rician fading channels for a binary differential phase‐shift keying modulation scheme. Analytical expressions for the probability density function (PDF) and moment‐generating function (MGF) are derived. The average bit error probability can be calculated by averaging the conditional bit error probability over the PDF or using the MGF‐based approach. Numerical results are graphically presented to show the effects of macrodiversity, correlation, number of diversity branches, and severity of both fading and shadowing.